This invention relates to a method of making piston rings, particularly for internal-combustion engines, by cutting individual rings from out-of-round metal sleeves.
Piston rings for internal-combustion engines have been manufactured with widely varying methods dependent on the eventual use. Compression piston rings or oil scraping rings are generally made of cast iron or steel. Dependent upon the expected stress on the piston ring, special alloys with suitable wear resistance, elasticity, strength and temperature resistance have been used. The piston rings are cast in an individual or a stacked casting process, or cylindrical or out-of-round sleeves are cast from which the individual rings are cut.
The sleeves are manufactured in various ways. With a centrifugal casting process only cylindrical sleeves may be made. From the sleeves individual rings are cut which pass through a number of treating stations until they reach their finished condition. After cutting open the rings they have to be submitted to complex thermal and/or mechanical working in such a manner that the rings obtain the required internal stress to ensure that when placed in the engine cylinder, they lie against the cylinder wall with a radially outwardly oriented tension. Sleeves made by a sand casting process have the disadvantage of high manufacturing costs because a preparation of the sand mold, relatively long dwelling periods in the mold and a subsequent cleaning of the cast pieces are required for further machining.
For avoiding the above-outlined disadvantages, according to German Patent No. 3,623,122 the sleeves are manufactured in a continuous casting process. Sleeves made in this manner excel in their fine grained, shrinkage cavity-free structure. These sleeves too, however, have to be submitted to various stations for further processing. In one machining station the individual rings are severed from the sleeve. Thereafter the rings are, in a suitably oriented manner, clamped together to form a stack and are cut open. The rings may be subsequently tensioned on a mandrel in a circular configuration and in a further station the rings are provided with a low-friction, wear-resistant coating on their outer circumferential surface.
German Offenlegungsschrift (application published without examination) 3,144,335 discloses a method of making a wear-resistant layer on the slide faces of piston rings. The individual rings are clamped together on a mandrel as a stack and are coated with a thermally sprayed layer. The thus-coated, mandrel-supported rings are thereafter advanced to a further processing station in which the individual rings are separated from one another. Thus, a substantial number of processing steps have to be performed which render the manufacture of the piston rings expensive.